1. Field of the Invention
The present invention relates to a liquid ejection head and an image forming apparatus, and more particularly, to a liquid ejection head and an image forming apparatus for which nozzles are disposed in high-density.
2. Description of the Related Art
An inkjet recording apparatus (inkjet printer) is known which performs so-called inkjet image recording in which an image is recorded by forming dots on a recording medium by ejecting liquid droplets (ink droplets) from a plurality of nozzles formed in a print head (also simply called “head”) while moving the head and the recording medium relatively with respect to each other.
Ink ejection methods in a head include: a thermal method in which heating elements (electrical-thermal energy conversion devices) are provided in the vicinity of the nozzles, the ink being heated locally by applying an electrical signal to each heating element, thereby creating a pressure change which causes ink droplets to be ejected from the nozzle; and a piezoelectric method in which electrical-pressure conversion devices, such as piezoelectric elements, are used to apply a mechanical pressure to the ink, thereby causing ink droplets to be ejected from the nozzle.
In order to make the head using the piezoelectric method possible to eject a prescribed liquid droplet (volume), the piezoelectric body constituting each piezoelectric element must have a prescribed surface area. For example, in order to eject a liquid droplet of several picoliters (pl), it is necessary to adopt a design in which the surface area of the piezoelectric body is approximately 0.1 mm2 to several tenths of 1 mm2. Therefore, the density of the nozzles which can be arranged in one row in the piezoelectric type of head is low compared to the thermal type of head, and is 180 nozzles per inch (npi), for example.
Then, heads in which nozzles are arranged in a two-dimensional configuration (matrix array) have been proposed (see, for example, Japanese Patent Application Publication Nos. 2001-334661, 2002-166543 and 2000-79683). The planar shape of the piezoelectric bodies is substantially rhombic or square, and by arranging a plurality of nozzle rows of medium density of the order of 30 npi, a high effective nozzle density (the nozzle density of the projected nozzle row obtained by projecting the nozzles in the direction of the nozzle rows), for example, 1200 npi to 2400 npi, is achieved.
However, the heads in the related art have problems as follows.
FIG. 20 is a plan view perspective diagram showing an example of a head in the related art. As shown in FIG. 20, the head 150 includes nozzles 151, pressure chambers 152 corresponding to the nozzles 151, and piezoelectric elements 158 having substantially the same shape as the pressure chambers 152, arranged in a two-dimensional configuration following the main scanning direction and an oblique direction which is not perpendicular to the main scanning direction. In this case, the projected nozzle row obtained by projecting the nozzles to an alignment in the main scanning direction is arranged at a uniform nozzle pitch P0. On the other hand, with regard to the nozzles 151 that are mutually adjacent in the projected nozzle row, the nozzle pitch is P1 in the sub-scanning direction between nozzles 151 that are aligned in the oblique direction (for example, between nozzles 151-11 and 151-12 and between nozzles 151-15 and 151-16, in nozzle row 154-1, and so on), whereas the nozzle pitch is P2, which is greater than P1 (i.e., P2>P1), in the sub-scanning direction between the nozzles 151 situated at the junctures (return positions) between nozzle columns 154 that are adjacent in the main scanning direction (for example, between the nozzle 151-16 of the nozzle column 154-1 and the nozzle 151-21 of the nozzle column 154-2).
In order to achieve high nozzle density in the head 150, as stated previously, the surface area of the piezoelectric bodies constituting the piezoelectric elements must be set to a prescribed size, and hence the size of the head 150 in the sub-scanning direction is inevitably enlarged. More specifically, the nozzle pitch P1 in the sub-scanning direction becomes larger, and consequently, the nozzle pitch P2 in the sub-scanning direction at the junctures also becomes larger. Consequently, there is a problem in that, if there is error in the installation position of the head, skewing of the paper feed direction or contraction of the recording medium due to cockling, or the like, then streaks extending in the paper feed direction (sub-scanning direction) are readily visible in the image formed on the recording medium around the positions corresponding to the junctures in the head.
It has been proposed that streaks occurring around the positions corresponding to the junctures can be made inconspicuous by adjusting the nozzle arrangement in the head suitably, or the like, but this gives rise to restrictions, for instance, it makes the flow channel structure inside the head more complicated.